A solid oxide fuel cell includes an air electrode to which air containing oxygen is supplied from the outside, a fuel electrode to which fuel gas such as hydrogen, carbon monoxide or methane is supplied from the outside, and an electrolyte through which oxygen ions can pass between these electrodes, and is constituted to generate electric power by electrochemical reaction of fuel gas and oxygen ions. At the air electrode, electrons supplied from an external circuit react with oxygen in air so that oxygen ions are produced. Then, the oxygen ions pass through the electrolyte to reach the fuel electrode. Further, at the fuel electrode, the oxygen ions having passed through the electrolyte react with fuel gas so that products are emitted and electrons are supplied to an external circuit.
As one conformation of this solid oxide fuel cell, there is conventionally a flat plate type. In the flat plate type solid oxide fuel cell, a fuel electrode film is formed on one surface (for example, a front surface) of an electrolyte plate, and an air electrode film is formed on the other surface (for example, a back surface), thereby forming a single cell. Then, a plurality of single cells are superposed each other with a separator plate having a gas flow path formed thereto being sandwiched therebetween, thereby forming a laminated body. Furthermore, when a metal manifold plate for distributing gas in accordance with each electrode is attached around the laminated body, a cell stack is formed.
In this flat plate type solid oxide fuel cell, however, since the structural strength of the cell stack is assured by the electrolyte plate and the separator plate, the extremely high accuracy of the dimension or flatness of the electrolyte plate or the separator plate is demanded in order to prevent the cell stack from being damaged due to the thermal stress generated during heating such as power generation. The processing cost of the electrolyte plate and the separator plate is therefore increased, and the quality control of these plates must be performed strictly.
Moreover, since the separator plate and the single cell are superposed as separate members, the electric resistance between these members becomes large, and large losses are generated in the electrical output from the stack.
In addition, since the separator plate has flow paths between respective electrodes by providing, e.g., ribs in order to distribute gas, the separator plate does not have enough strength and is apt to be damaged. Additionally, its processing cost is increased.
Further, since a material used for the separator plate is relatively expensive and must have an enough thickness in order to assure the strength, the material cost of the cell stack is increased.
Furthermore, since the manifold plate provided around the laminated body constituted by superposing the single cells is made of metal, the thermal expansion coefficient of the manifold plate is greatly different from that of the laminated body of the single cells. Therefore, the thermal stress is produced between the laminated body and the manifold plate during electric power generation, which can be a factor of damage to the cell and stack.
It is an object of the present invention to provide a single cell of a flat plate type solid oxide fuel cell and a cell stack utilizing this single cell which can improve the strength of the cell and/or stack and facilitate processing of components to reduce the processing cost. Moreover, it is another object of the present invention to provide a single cell of a flat plate type solid oxide fuel cell and a stack utilizing this single cell which can improve the output characteristic and reduce the material cost.